If you want to receive a response from a service, you should know the type of service - sync or async. "Reply(Sync)" and "Callback(ASync)" are used to send back the response to the caller. Hence, you'll use "Invoke( If external service is Sync)" or "Receive (If external service is ASync)" to get the response.

You can read about activities in deep and experiment via practical examples to make things clear.